This study assessed the impacts of differing levels of land development in four watersheds in Palau on river sediment yield and on sedimentation and turbidity. Area corrected sediment yield was strongly related to land development ( , ), varying from 9.7 to 216？tons km？2 yr？1 between the least and most developed watershed. Mean sedimentation rates on reefs ranged from 0.7 to 46？mg cm？2 d？1, and mean turbidity ranged from 9 to 139？mg l？1. The higher values exceeded those known to harm corals. Because Palau's watersheds and estuaries are small, river floods were short-lived (typically lasting less than a day) and the estuaries adjusted just as quickly to a number of different estuarine circulation patterns that, in turn, generated a large variability in the export of riverine fine sediment to the reefs. The ultimate fate of the fine sediment deposited on the reefs depended on wind resuspension, local currents, and geomorphology (whether the bay was open or semi-enclosed). Palau's small estuaries were generally not as effective as bigger estuaries in trapping sediments and thus at sheltering the reefs. Therefore, greater efforts are needed to control and mitigate land activities that contribute to the increase in sediment yield. 1. Introduction Coral reef ecosystems include some of the most diverse biological communities on earth, and like other ecosystems, are being lost due to anthropogenic disturbance. Approximately 20% of the world’s coral reefs are already severely degraded, with another 24% under imminent risk and 26% expected to be lost within the next several decades [1]. Documented losses include taxonomic diversity, genetic diversity, elements of ecosystem structure and function, resilience to disturbance and ecosystem services. The major human-induced stressors affecting coral reefs include exploitation of resources (including overharvesting of herbivorous fishes that control algal populations), global climate change responsible for mass-bleaching events and ocean acidification, and land-based sources of pollution tied to increased levels of erosion and sedimentation from the modification of adjacent watersheds. Sedimentation of coastal environments is a major issue worldwide, with most of the increase attributed to land clearing for agriculture and other activities that disturb the land surface [2]. A worldwide analysis of high sediment areas shows that coral reefs are less likely to be found near areas with naturally high terrestrial runoff [3]. For those areas that do have coral reefs, 22% of them face medium to high threats from increased
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